The present invention relates to a metering circuit for detecting the presence of electric current in a metered circuit, furnishing at the same time insulation between the metering circuit and the metered circuit where it is desired to detect the current.
Consequently, the circuit of the invention is applicable to auxiliary telephone equipment, such as markers, answering devices, etc., which are controlled by a telephone connected to the line. The on-hook and off-hook conditions of the telephone are detected by the metering circuit which gives orders to the auxiliary equipment.
The detection system or metering circuit of the invention is based on the property of magnetic materials of becoming saturated if the excitation applied is excessive.
As will be discussed later in more detail, the proposed circuit includes the primary coils of two equal transformers, which primary coils are mounted in series. The secondary coils of the transformers are mounted in series opposition with each other and in series with the metered circuit in which the current is desired to be detected.
The transformer primary coils constitute, in accordance with the invention, the load of the collector of an NPN transistor, which has its emitter connected to a negative terminal through a resistor-condenser cell, and its base, polarized through a resistive divider, fedback from its collector, through two condensers in series which, at their point of union, are connected to a positive terminal with the interposition of a resistor.
Lastly, and as will be made evident below, the collector of the NPN transistor is connected also, through a resistor, to the base of a PNP transistor which has its emitter connected to the positive terminal and its collector to the negative terminal, this last connection through a resistor-condenser unit.